Treatment Process for Bars

ABSTRACT

Treatment process for stainless steel bars, in particular a solution quenching, to be performed directly in-line downstream of the rolling mill which makes it possible to obtain a material devoid of intergranular corrosion and with microstructural characteristics suitable for subsequent uses. Advantageously, said process also makes it possible to improve the productivity of the entire rolling plant. The treatment is suitable to be performed on austenitic, ferritic, or austeno-ferritic stainless steel bars, Al—Cu alloy bars, Nickel alloy bars, and all other alloys requiring rapid cooling in order to prevent undesired phase precipitations. Prevention of intergranular corrosion obtained with the treatment process of the invention makes it possible to prevent problems, and relative costs, during surface treatment of the bars and those that could occur in final use.

FIELD OF THE INVENTION

The present invention relates to an in-line treatment process of rolledbars, in particular to a solution quenching treatment, at the outlet ofthe rolling train, of bars in stainless steel, nickel alloys or otheralloys that require rapid cooling after rolling.

STATE OF THE ART

The structure of austenitic steels is practically composed only ofaustenite stable at ambient temperature; the chromium carbides, whichmay be held in supersaturated solid solution in the austenite, arecapable of precipitating, at the austenite grain boundary, when thesteel is heated in the interval ranging from 450 to 850° C. or cooledslowly through this interval. The considerable carbide-forming power ofthe Cr causes disproportionation of the alloy at the grain boundary,while in the centre of the grains the Cr content remains practicallyunchanged. In the case in which the content of free Cr, that is, notbonded to form carbides, drops locally below the limit of stainlessness(12%), the steel becomes susceptible to inter-granular attack. Attack byintergranular corrosion can cause disbonding of a grain with respect tothe others, with a considerable decrease in mechanical resistance and toimpacts, notwithstanding a negligible loss in mass.

As they are well known in literature, further details will not be givenhere on steels and on the problems related to the effects of an attackof intergranular corrosion.

To prevent problems of intergranular corrosion, action is taken on themetal either through heat treatment carried out later, off-line, or bymodifying the chemical composition, so that the carbides of Cr or ofother secondary phases do not precipitate. The methods used comprisedthe following:

-   a decrease in the content of C,-   use of stabilizing elements, such as Ti, Nb, V and Ta, together with    an off-line stabilization treatment.

Solution quenching, also referred to as “hyperquenching” or “negativequenching”, is intended as a rapid cooling treatment to preventprecipitation of chromium carbides of other secondary phases. The onlything this treatment has in common with conventional quenching is thecooling speed from the austenization temperature, and not the structuraland hardening effects which normally occur for hardenable steels.

As known, during off-line solution quenching, by a re-heating at hightemperature the chromium carbides precipitated at the grain boundaryduring heat treatment, or when the metal was held at a temperature inthe critical interval of carbide precipitation, or cooled slowly throughthis interval, are dissolved. Therefore, solution heat treatment makesit possible to increase the resistance to intergranular corrosion bydelivering a material without areas that are deficient in chromiumcontent and therefore immune to intergranular corrosion.

Normally, the temperature interval in which the solution heat treatmentis performed ranges from 950 to 1200° C., more frequently from 1000 to1100° C., with holding times of around 1 minute per mm of thickness.

Therefore, the treatment is performed at the lowest temperaturesufficient to solubilize the carbides and to eliminate the degree ofstress caused by the various processing cycles, while the permanence atthis temperature is strictly related to the thickness of the producttreated and must be kept to the minimum times required.

Higher temperatures and longer holding times can, in fact, cause morepronounced softening and, depending on the parameters adopted, alsopromote undesirable enlargement of the grains, which is needed forcertain specific applications.

To prevent precipitation of the chromium carbides, austenitic stainlesssteels must be cooled rapidly from the solution treatment temperatures.

In a bar rolling plant, the off-line process consists in quenching, thatis, immersing the rolled bars in special tanks of water or suitablefluids. These tanks are usually small in size and allow the treatment oflimited length bars, drastically reducing the yield of the plant andoverall productivity. In this case, the bars are cooled to below 300° C.

If the treatment is performed outside the line, it has the disadvantageof requiring intermediate storage and re-heating from ambienttemperature to a temperature of around 1100 to 1200° C., withconsiderable energy consumption as well as consumption of material andmaintenance. Moreover, the bars must be straightened after quenching inthe tank, even simply to be transported and stored. All this requiresconsiderable overall dimensions of the plant, high maintenance and leadsto low productivity.

The in-line treatment provides quenching after hot rolling in a quenchtank of water or suitable fluids that can be set up downstream of therolling mill.

In this case, the bars must be straightened after quenching in the tank,even simply to be transported and stored. All this requires considerableoverall dimensions of the plant, high maintenance and leads to lowproductivity. Furthermore, it is not always possible to guarantee thatthe temperatures at the beginning of the treatment are the optimaltemperatures, both for obtaining a material without carbides, and forobtaining a homogeneous microstructure in terms of size of theaustenitic grain.

SUMMARY OF THE INVENTION

The present invention makes it possible to perform the solutionquenching on bars with round, square, hexagonal, rectangular section,directly at the outlet of the rolling mill, and to obtain a high qualityproduct that does not require straightening and is free of scoring andsurface defects. The process is performed in a compact plant, withoutspecific maintenance requirements, which allows to eliminate all costsrelated to intermediate storage of the material, as well as thoserelated to reheatings in furnace, which are necessary for normaloff-line treatments. This highly flexible system makes it possible topass from producing one shape to another one without having to performmechanical intervention with the related loss of time.

The main advantages of this invention are due to:

1) performing the solution quenching in-line with the rolling process;

2) efficiency of the plant and reduction of the use of equipment andmaintenance;

3) increase of the global productivity of the plant;

4) elimination of straightening after the treatment;

5) drastic reduction in surface defects;

6) elimination of costs linked to intermediate storage and reheating;

With regard to the above points, we stress that:

1) A primary object of the present invention is to produce a treatmentprocess for stainless steel bars, in particular a solution quenchingtreatment, directly in line downstream of the rolling mill, which makesit possible to obtain a material devoid of intergranular corrosion andwith microstructural characteristics suitable for subsequent uses. Infact, the use of austenitic stainless steels is based on theirstructure, and even minimum microstructural variations may have aconsiderable influence on the corrosion behaviour of the final productin different environments. Moreover, these structural variations canhave enormous importance, not only in relation to final application, butalso in relation to the surface treatments to which the product must besubjected: in fact, corrosion phenomena could also occur during thesesurface treatments.

2) The high efficiency of the heat exchange achieved in the coolingdevices, or simply quenching tanks, with water or other suitable fluidsallows a reduction of the dimensions and amount of equipments necessary,obtaining a particularly compact plant which is capable of providingcompletely solubilized material up to diameters of 130 mm and equivalentsections. The possibility of extending the treatment to larger diametersdepends on the speeds adopted. The reduction of the devices used andtheir simplicity means that less maintenance is necessary.

3) A further object of the invention is to perform the treatment so asto improve the productivity of the entire rolling plant. It is possibleto process bars of any length, and this depends only on the size of thefinal cooling device.

4) The design of the quenching tanks with water or other suitable fluidsand the operating parameters adopted permit to achieve not only a highheat exchange but also an optimal uniformity of cooling which guaranteesthe straightness necessary to eliminate the straightening of the barsafter treatment, contrarily at the bars treated in the traditional waywhich have to be straightened after quenching in tank, even simply to betransported and stored.

5) The devices used for the treatment also allow a drastic reduction insurface defects, improving the overall quality of the finished product.

6) The possibility of treating the bars directly in line, at the outletof the rolling train, avoids intermediate storage and re-heating fromambient temperature to temperatures of around 1100 to 1200° C. withevident energy saving. This treatment is suitable to be performed onaustenitic, ferritic or austeno-ferritic stainless steel bars, Al—Cualloy bars, Nickel superalloy bars, and bars of any other alloy whichrequires rapid cooling.

Therefore, the present invention proposes to achieve the aforementionedobjects by providing an in-line heat-treatment process of bars exitingfrom a rolling mill in which, according to claim 1, there is providedimmediately after at least a first rolling stage a solution quenchingtreatment in a solution quenching line, with a cooling speed such that afirst curve, representing bar core temperature trend versus time, doesnot intersect and stays below a second precipitation curve of secondaryphases, tipical for the bar material.

The dependent claims describe preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE FIGURES

Further characteristics and advantages of the invention shall be moreevident in the light of the detailed description of a treatment processfor bars, of which some results are shown by way of a non-limitingembodiment, with the aid of the accompanying drawings, wherein:

FIG. 1 shows the temperature trend through the section of a first barduring rolling and the treatment of the invention;

FIG. 2 shows the temperature trend through the section of a second barduring rolling and the treatment of the invention;

FIG. 3 shows the temperature trend through the section of a third barduring rolling and the treatment of the invention.

FIGS. 4, 5, 6, 7, 8, show some configurations of a rolling plant that issuitable to perform the treatment in-line of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

To determine the treatment process of the invention, which makes itpossible to obtain, directly after rolling, a material devoid ofintergranular corrosion and with microstructural characteristicssuitable for subsequent uses, it was fundamental to identify thefollowing parameters:

-   the position of the devices adapted to guarantee the correct    treatment;-   the initial treatment temperature interval to obtain a structure    totally devoid of carbides and with a homogeneous microstructure;-   the end of treatment temperatures, that is, those for which passing    through the critical range of carbide precipitation can be    considered to have terminated, and for which distortion due to    residual heat is avoided.

The duration of the treatment according to this invention is a parameterwhich depends on the dimensions of the bar and on the rolling speed.

Other important parameters to perform the solution quenching treatmentin line, directly downstream of the rolling mill, comprise:

-   rolling parameters, such as speed, reduction ratios and temperature,    suitable to guarantee an initial austenitic structure and,    therefore, an optimal grain dimension for subsequent uses;-   the speed at which the rolled bar is fed through a cooling device to    guarantee the minimum times required to perform the desired thermal    gradient;-   the necessary flow rates and pressures of the cooling fluid,    generally water, inside the device in order to guarantee a heat    exchange coefficient that is sufficient to obtain the minimum    thermal gradient required.

FIGS. 1 to 3 show the experimental results related to the trend of theparameter temperature for the treatment according to the invention.

The diagram in FIG. 1 represents the temperature trend of a first AISI304 austenitic steel bar, along a cross-section thereof, while it isbeing fed through the entire rolling plant. In particular, the curve 1represents the temperature trend of the centre of the bar, also definedas core, along a cross section, orthogonal to the longitudinal axis ofsaid bar; the curve 2 represents the temperature trend in anintermediate surface; the curve 3 the trend of the outer surface of thebar. The positions of the various rolling stands present in theproduction plant, used for these tests, are indicated on the horizontalaxis X. It can be seen how, at the outlet of the last stand 4 of therolling train, solution quenching treatment of the bar commences, byfeeding it through a suitable cooling device, with a cooling speed thatallows a temperature trend of the core of the bar that does intersectand that remains below the precipitation curve 5 of the carbides, orsensibilization curve. This curve 5 is a function of the chemicalcomposition of the material of which the bars are composed and alsodepends on the dimension of the grain and, therefore, on the section ofthe bar being treated. Precipitation curves are easy to find inspecialized literature. This first bar, relative to FIG. 1, is fed tothe rolling train with a dimension of 41 mm and is reduced to adimension of 27 mm with a rolling speed of 4.9 m/s. The initial rollingtemperature is between 1100° C. and 1150° C. The temperature of the barat the start of solution quenching treatment ranges from 1050° C. to1100° C., while at the end of treatment it ranges from 350 to 500° C.The treatment time ranges from 4 to 6 seconds, in order to preventcarbide precipitation and to obtain the required straightness, while thepressure of the cooling fluid is around 4-8 bar.

Analogously, the diagram in FIG. 2 represents the temperature trend,during the treatment of the invention, through the section of an AISI304 steel bar with a diameter of 86 mm, obtained by rolling with a feedspeed of 0.7 m/s starting from an initial rolling temperature of between1130 and 1180° C. The initial treatment temperature of the inventionranges from 950 to 1000° C., while at the end of treatment it rangesfrom 500 to 600° C. The treatment time suitable to prevent carbideprecipitation is in this example of about 35 to 40 seconds, while thepressure of the cooling fluid is around 4-6 bar.

The diagram in FIG. 3, instead, refers to an AISI 304 stainless steelbar with a diameter of 60 mm, obtained by rolling with a feed speed of1.2 m/s starting from an initial rolling temperature of between 1130 and1180° C. In this case the initial treatment temperature ranges from 980to 1020° C., while at the end of treatment it ranges from 450 to 600° C.In this case the treatment time ranges from 18 to 24 seconds in order toprevent carbide precipitation and to obtain the required straightness;the pressure of the cooling fluid is around 4-8 bar.

In all three examples in FIGS. 1 to 3 it was possible, therefore, toobtain by means of the solution quenching treatment of the invention,performed directly on the rolling line, a material with a highresistance to intergranular corrosion. This is due to the choice oftreatment parameters within specific intervals which prevent undesiredphase precipitations such as those of the chromium carbides. Thesolution quenching stage of the invention is suitable to be performedalso on ferritic or austeno-ferritic stainless steel bars, Nickelsuperalloy bars, and all other alloys that require rapid cooling.

The treatment of bars of the invention is suitable to treat products inbars with a diameter ranging from 10 mm to approximately 130 mm, and theequivalent in square, hexagonal or rectangular sections, said treatmentbeing characterised by the following parameters:

-   an initial treatment temperature ranging approximately from about    950 to 1200° C.;

an end of treatment temperature ranging from 350 to 650° C.

Advantageously, the cooling speed is between 10 and 150° C./sec.

The cooling device that allows implementation of the in-line solutionprocess of the invention consists in a series of cooling boxes, housingpipes with pressurized water, or suitable fluids, and with adistribution of the jet of water, or suitable fluids, suitable toguarantee uniform cooling of the entire section of the bar, avoidingdistortions of the bar typical of rapidly cooled austenitic steels.

A number of cooling boxes is necessary to guarantee the thermal gradientrequired by the treatment of the invention. The thermal gradient, whicha cooling box must produce on the bar, varies as a function of thediameter of the bar, the grade of the steel and the end treatmenttemperature required. There is provided also a system to support thebar, suitable to ensure the absence of surface scoring and to correctlycentre the bar to achieve uniform cooling.

With the treatment process of the invention, performed directly duringrolling, it is therefore possible to avoid intermediate storage andre-heating from ambient temperature to a temperature of around 1100 to1200° C. with evident energy saving; it is no longer necessary tostraighten the bars after the cooling; productivity is the nominal plantproductivity as the process is performed directly during rolling; theentire production plant is more compact and requires less maintenance.

FIGS. 4, 5, 6, 7, 8 show some of the possible solutions for theconfiguration of the plant that allow the in-line execution of thestages of rolling and solution quenching treatment in which the total orpartial cooling is carried out in boxes of water or other suitablecooling fluids.

FIG. 4 gives a schematic illustration of an initial basic configurationof the plant, comprising:

-   a reheating furnace 6 for semi-finished products, such as billets,-   a roughing train 7, an intermediate train 8 and a finishing train 9    which define respectively the first, second and third stage of    rolling the bars,-   a first solution quenching line 10 comprising a series of cooling    boxes with water or other suitable fluids,-   a discharging device 11 for the bars.

Before the quenching treatment in the first solution quenching line 10,the bars can also undergo just one stage of rolling in the roughing rain7, or just two stages of rolling in the roughing 7 and intermediate 8trains.

The configuration in FIG. 5, meanwhile, shows how the bar is cooled in asecond solution quenching line 10′, placed immediately downstream of thefirst stage of rolling, and therefore of the roughing train 7; it alsoshows that the bar is discharged into an intermediate bar dischargingdevice 11′. The remainder of the plant is as that described for FIG. 4,from the second stage of rolling in the intermediate train 8 andcomprising said first solution quenching line 10. This configuration isparticularly advantageous because it makes it possible to better controlthe temperature at the beginning of the treatment for those bars thatonly require the first stage of rolling: indeed, it avoids these barsremaining for too long in the air throughout the other rolling stages,which would not be activated, before reaching the entrance of thesolution quenching line. Furthermore, this configuration helps theoperations on the section of the line downstream of the intermediate bardischarge device 11′, in the sense that it is possible to simultaneouslyproduce and carry out maintenance or changes on said section; this makesthe plant more flexible. This configuration is suitable to be suggestedto the user when installing a new plant.

In the configuration in FIG. 6, unlike that in FIG. 5, there is nointermediate discharge device 11′, leading to greater compactness of theplant; thus it is possible to reduce it by around 30 m in length.

The configuration in FIG. 7, like that in FIG. 5, makes it possible todischarge separately those bars that undergo just one stage of rolling.In this case, there are advantageously two parallel lines, whichcomprise respectively the first line 10 and a second line 10″ ofsolution quenching, which are fed by a device 13 that transfers thebars, placed downstream of the first stage of rolling. Downstream of thesecond line 10″ there is provided a second discharge device 11″. Thisconfiguration is suitable to be incorporated into a plant that alreadyhas a solution quenching line, such as that illustrated in FIG. 4.

Lastly the configuration in FIG. 8 provides that the solution quenchingtreatment is carried out in two stages: the first stage is carried outin the first solution quenching line 10, placed downstream of thefinishing train 9, and the second stage is carried out in a water tank12 which is integrated into the bar discharge device 11; the advantageof this configuration lies in the fact that the length of the solutionquenching line 10, in which the first stage is carried out, is shorter,for example, than the equivalent length in the configuration in FIG. 4,further increasing the compactness of the plant.

The particular methods of embodiment described herein do not limit thecontent of this application, which covers all embodiments of theinvention defined by the claims.

1-14. (canceled)
 15. In-line heat treatment of bars exiting a rollingmill wherein, immediately after at least one stage of rolling, there isprovided a solution heat-treatment in a solution heat-treatment line(10, 10′, 10″), with an initial temperature of the bars comprisedbetween 900 and 1200° C., characterised in that there is provided aquenching speed comprised between 10 and 150° C./sec, and in that thefinal temperature of the solution heat-treatment of the bars is between350 and 650° C., such that a first curve (1), which represents thetemperature pattern at the core of the treated bar over time, does notintersect and remains below a second curve (5) showing precipitation ofcarbides, specific to the material of the treated bar.
 16. Processaccording to claim 15, wherein said bars are of circular, square,hexagonal, rectangular section.
 17. Process according to claim 16,wherein said bars are in stainless steel.
 18. Process according to claim16, wherein said bars are in nickel alloys.
 19. Process according toclaim 15, wherein two stages of rolling take place before the solutionheat-treatment in said first solution heat-treatment line (10), saidstages including the passage of the bars to a roughing train (7) and anintermediate train (8) respectively.
 20. Process according to claim 15,wherein three stages of rolling take place before the solutionheat-treatment in said first solution heat-treatment line (10), saidstages including the passage of the bars to a roughing train (7), anintermediate train (8) and a finishing train (9) respectively. 21.Process according to claim 15, wherein said at least one stage ofrolling includes the passage of the bars into a roughing train (7)downstream of which the bars undergo said solution heat-treatment in asecond line of solution heat-treatment (10′, 10″).
 22. Process accordingto claim 19, wherein said solution heat-treatment includes two stages,the first of which on said solution heat-treatment line (10) and thesecond of which in a quenching medium (12) that is integrated into afirst bar discharge device (11).
 23. Process according to claim 20,wherein said solution heat-treatment includes two stages, the first ofwhich on said solution heat-treatment line (10) and the second of whichin a quenching medium (12) that is integrated into a first bar dischargedevice (11).
 24. Process according to claim 19, wherein after saidsolution heat-treatment the bars are discharged into a first dischargedevice (11), called first device (11) as it is situated immediatelydownstream of said first solution heat-treatment line (10).
 25. Processaccording to claim 20, wherein after said solution heat-treatment thebars are discharged into a first discharge device (11), called firstdevice (11) as it is situated immediately downstream of said firstsolution heat-treatment line (10).
 26. Process according to claims 15 or21, wherein after said first stage of rolling there is a stage in whichthe bars are transferred to a second solution heat-treatment line (10″).27. Plant for rolling and in-line heat treatment of bars including meansfor heating (6) a semi-finished product, means for rolling saidsemi-finished product, including a roughing train (7), an intermediatetrain (8) and a finishing train (9), a first solution heat-treatmentline (10) including a series of quenching tanks with water or otherquenching fluids, a first bar discharge device (11) equipped with aquenching tank (12).
 28. Plant according to claim 27, wherein betweensaid roughing train (7) and said intermediate train (8) there is asecond line of solution heat-treatment (10′).
 29. Plant according toclaim 28, wherein immediately downstream of said second solutionheat-treatment line (10′) there is a second bar discharge device (11′).30. Plant according to claim 27, wherein downstream of the roughingtrain (7) there is a device (13) to transfer the bars to a secondsolution heat-treatment line (10″) followed by a second discharge device(11″).